bam_model_extensions changes external path references in distributions

[izzyboris]

Description

If a model file is converted using the bam_model_extensions from #714 and it references any external resources by pathname (textures, etc.), the write_bam_file() that saves the file changes the relative paths of these models and they may point into the source directory. Ideally I think the relative path references in the .bam outputs from the converter should be left alone so they point to the texture relative to their current location.

Steps to Reproduce

• Create a model with an external texture (e.g., a .blend using blend2bam or .gltf)
• Put ".blend" or .gltf in the bam_model_extensions list
• Run bdist_apps
• The external texture path will get munged during the file write_bam_file() within commands.py:_model_to_bam and point at the source directory instead of a texture copied into the build directory.

Environment

• Operating system: Windows (probably happens in Linux too)
• System architecture: 64-bit Intel
• Panda3D version: 0.10.13
• Installation method: pip
• Python version (if using Python): 3.11
• Compiler (if using C++): N/A

[rdb]

Ah, yes, the default bam-texture-mode is "relative", which is trying to be clever and keep the paths relative, but that makes them point into the source directory.

As a workaround, set bam-texture-mode to "unchanged" in your Config.prc. I will make build_apps do this automatically.

Ideally, it would actually keep it to "relative", but make them relative to the target directory, so that absolute paths are converted.

[rdb]

Noticing in my tests that the file got absolute paths with "unchanged", I made the mode always forced to "relative" but changed the path they were being made relative to.

To accommodate people who might have been previously setting bam-texture-mode rawdata, I added a new bam_embed_textures option. If you set it to true, the textures will be embedded into the .bam files.